In 2011, California told pop makers the state would slap warning stickers on their products unless they reduced the level of a chemical called 4-methylimidazole (4-MEI) to less than 29 micrograms per can or bottle.

That year, the American Center for Science in the Public Interest warned Health Canada and the U.S. Food and Drug Administration (FDA) to bar the chemical.

"Barring the use of the caramel colourings produced with ammonia could prevent cancer in thousands of consumers," the centre wrote.

But while Pepsi and Coca-Cola quickly made changes in California, some Pepsi products tested above the threshold in New York last month, according to tests published this week by Consumer Reports.

Any changes made by Coca-Cola in the U.S. have also been made in Canada, a spokesman said.

But Pepsi won't complete the switch in the U.S. until February and won't start in Canada until March.

PepsiCo defended its actions, saying the chemical additive doesn't run afoul of any regulation.

"Health Canada and other regulatory agencies around the world, including the U.S. Food and Drug Administration and European Food Safety Authority, consider our current caramel colouring safe for use in foods and beverages," Gonzalez wrote.

Since the change had to be made in California, it made sense to change it elsewhere rather than have two lines of the same products, she wrote.

But while the FDA still considers the chemical safe, data from Consumers Reports has prompted a second look by the agency.

"The FDA appreciates the work done by Consumer Reports... The agency is also reviewing new data on the safety of 4-MEI," The agency wrote to QMI Agency.